1. Field of the Invention
The present invention relates to an improvement in a power steering device for vehicles, and in particular to a power steering device provided with a hydraulic motor type speed sensor.
2. Description of the Prior Art
Generally, in a power steering device for vehicles, when a hydraulic power source breaks down, a steering system is automatically switched over to a manual steering system to permit safe driving. However, this method is not necessarily satisfactory for a comparatively heavy vehicle such as a large passenger car or truck because there is no power assistance after the steering is switched over to a manual steering system, and an extremely large manual force is required for steering such a heavy vehicle. Thus, smooth steering is hardly possible, which is undesirable from the view point of the safety of the driver.
There has been suggested a system having two power sources with an auxiliary hydraulic pump which is normally idly driven with the engine power and is alternatively switched over to serve as a substitute hydraulic power source when a main hydraulic power source breaks down. However, such system requires two entirely separate hydraulic pumps and necessarily results in increase of costs. Further, even with a system having two power sources, in case the engine stops, both power sources will stop operation at once and the auxiliary hydraulic pump and its channel will become useless.
As an improved system for the above, it has been considered to drive the auxiliary hydraulic pump with a shaft such as a counter-shaft of a transmission, which rotates at the number of rotations proportional to the vehicle wheel, instead of the engine. However, in case the main pump and channel are complete, power loss will be caused with driving of the auxiliary pump. Also, because the auxiliary pump is always driven, the oil temperature will be raised, to thus deteriorate the hydraulic oil.
In such a power steering device system having two power sources, presupposing trouble with the main power source, it is not sufficient that only the remaining power source superseding the broken down power source is used alone. Thus, it is further required to provide a system for detecting trouble with a power source and warning the driver of same. However, such warning system is required to be more reliable than the power steering system itself. Thus, if this warning system is attached to the two power sources system and the reliability is made high, the thus constructed power steering device will be so costly as not to be an economical and useful device in practice.
The present invention effectively solves the problems described above by taking into account the following considerations.
At the present, there are various kinds of systems for controlling the flow gain of the hydraulic oil with the change of the vehicle speed and adjusting the steering to be hard with the increase of the vehicle speed, which systems are incorporated in the conventional power steering devices. In general such systems drive a vehicle speed sensor by a shaft rotated at the number of rotations proportional to the vehicle wheel, such as a counter-shaft of a transmission. Therefore, if this speed sensor can be utilized as a second hydraulic power source so that, in case a main hydraulic power source breaks down, the speed senser may be automatically converted to a substitute power source, a desirable power steering device will be attained with ensuring the safety and reliability and satisfying economical requirements by utilizing the existing equipment.